1. Field of the Invention
The present invention relates to fiber optics, and more specifically, it relates to fiber optics that tailor the shape of a guided mode.
2. Description of Related Art
Conventional types of fiber optics that are used when a shaped optical mode is desirable include (i) large mode area fibers, (ii) large flat mode fibers, (iii) photonic-crystal fibers and (iv) Bragg fibers.
In large mode area (LMA) fibers, the refractive index profile is generally a single step, and the index difference between the core and cladding is made as small as manufacturing tolerances allow. The diameter of the fiber's core region is made large by telecommunications standards—generally greater than 20 μm. The large core allows the mode to spread over a large area, which is beneficial. Unfortunately, it also allows the fiber to guide many modes, and these degrade the shape of the guided beam. To mitigate the latter effect, end-users generally eliminate higher-order modes by bending the fiber. This works only to a point, though; as the core is made larger and larger, the fiber propagates more and more modes, and it becomes progressively more difficult to distinguish the desired mode from the growing number of undesired modes.
Large flat mode fibers are similar to LMA fibers, except that a ring of higher index glass is added at the outer edge of a step-like core. If properly designed and fabricated, the ring can stretch the mode from a bell-like shape to one having a rectangular-like shape. For a given number of guided modes, the rectangular shape reduces the peak field intensity in the fiber and thus reduces the threshold for nonlinear artifacts. Unfortunately, no such fiber has yet been successfully fabricated. Attempts have generally been limited by manufacturing tolerances; the limiting constraint now appears to be fluctuations in the refractive index of the step-like core that are on the order of 10−4.
In photonic-crystal fibers, also known as microstructured fibers, photonic bandgap fibers, and holey fibers, the effective index of the core, cladding, or both is tailored by changing the size and distribution of periodically placed holes within the fiber's structure. The manufacturing tolerances of these fibers allow for larger guided mode areas than can be reliably produced by more common manufacturing techniques, but unfortunately the fibers are today relatively fragile and expensive. Moreover, we are not aware of any designs to flatten the shape of the guided mode by this technique.
Bragg fibers date back to the late 1970's (Yeh, Yariv, and Moram), with more recent work by B. Temelkuran, Y. Fink, and coworkers in the late 1990's and early 2000's. Such fibers are not widely available, perhaps because their structures are much more complex than the others described here. Moreover, designs to flatten the shape of the guided mode in Bragg fibers are not known.